CST Strain Gauge & Piezoelectric Sensors 3 — Questions and Answers
Question 1: What phenomenon occurs when a piezoelectric crystal is mechanically stressed and generates an electrical charge?
- The magnetostrictive effect
- The direct piezoelectric effect (Correct answer)
- The converse piezoelectric effect
- The piezoresistive effect
Correct answer: The direct piezoelectric effect
The direct piezoelectric effect describes the generation of electrical charge in a crystal when mechanical stress is applied.
Question 2: A load cell has a rated output of 2 mV/V. With a 10 V excitation, what is the full-scale output signal?
- 2 mV
- 20 mV (Correct answer)
- 200 mV
- 0.2 mV
Correct answer: 20 mV
Full-scale output = rated output × excitation = 2 mV/V × 10 V = 20 mV.
Question 3: Which material is most commonly used as the piezoelectric sensing element in industrial force sensors?
- Quartz (SiO₂) (Correct answer)
- Silicon carbide (SiC)
- Barium ferrite
- Gallium arsenide (GaAs)
Correct answer: Quartz (SiO₂)
Quartz is the most widely used piezoelectric material in industrial sensors due to its stability, high modulus, and negligible pyroelectric effect.
Question 4: In a load cell with remote sense wiring (6-wire connection), what do the sense leads measure?
- The load cell output signal voltage
- The actual excitation voltage at the load cell terminals (Correct answer)
- The shield ground reference
- The temperature at the load cell
Correct answer: The actual excitation voltage at the load cell terminals
Remote sense leads allow the indicator to measure and compensate for voltage drop in the excitation supply leads, ensuring the load cell receives the correct excitation.
Question 5: What is 'hysteresis error' in a strain gauge load cell?
- The difference in output between increasing and decreasing load at the same load value (Correct answer)
- The change in output due to temperature fluctuation
- The slow drift in zero reading over time
- The nonlinearity between output and applied load
Correct answer: The difference in output between increasing and decreasing load at the same load value
Hysteresis is the difference in output readings at the same applied load when approached from increasing versus decreasing load directions.
Question 6: A load cell's zero balance specification is expressed as ±1% of rated output. For a 2 mV/V cell at 10 V excitation, what is the maximum allowable zero offset?
- ±0.2 mV (Correct answer)
- ±2 mV
- ±0.02 mV
- ±20 mV
Correct answer: ±0.2 mV
Full-scale output = 2 mV/V × 10 V = 20 mV; 1% of 20 mV = 0.2 mV maximum allowable zero offset.
Question 7: What is the primary purpose of a Zener diode barrier (zener barrier) when installed between an indicator and a load cell in a hazardous area?
- To amplify the load cell signal for long cable runs
- To limit the energy that can reach the hazardous area to intrinsically safe levels (Correct answer)
- To filter EMI from variable frequency drives
- To provide temperature compensation for the bridge circuit
Correct answer: To limit the energy that can reach the hazardous area to intrinsically safe levels
Zener barriers clamp voltage and limit current to prevent ignition-capable energy from reaching classified hazardous locations.
CST Strain Gauge & Piezoelectric Sensors 3
What phenomenon occurs when a piezoelectric crystal is mechanically stressed and generates an electrical charge?